Steel with Reduced Density and Method for Producing a Flat Steel or Long Steel Product from Such a Steel

ABSTRACT

The invention provides a density-reduced material based on iron, whose mechanical properties make it suitable for a wide range of applications. The steel has a density of less than 7.25 kg/dm 3  and includes (in wt %) C: up to 0.20%, Si: 0.1-3.50%, Mn: 0.1-3.50%, N: up to 0.020%, S: up to 0.40%, P: up to 0.009%, Al: 6.0-25.0%, Ti: 0.55-10.0%, Cr: up to 6.0%, Mo: up to 3.0%, Ni: up to 4.0%, V: up to 1.0%, W: up to 1.0%, Cu: up to 4%, B: up to 0.08%, Nb: up to 1.5%, remainder iron and unavoidable impurities caused during production. In this case, the structure of the steel has more than 85 vol % ferrite and up to 10 vol % austenite and as the remainder contents of intermetallic phases and proportions of carbide, nitride, bainite or perlite.

The invention relates to a steel having a reduced density as a result of its high Al content and a method for producing a flat or long product from such a steel.

When information is given in “%” in the present text in connection with alloy specifications or compositions of materials, this always relates to the weight. If, on the other hand, information on the proportions of determined structural parts is given, this always relates to the respectively observed volume.

“Flat steel product” or “flat product” are designated in the present text as rolled products, whose thickness is much lower than their length and width. The flat steel products or flat products in question are in particular sheets, strips or boards obtained from these sheets or strips.

The terms “long steel products” or “long products”, on the other hand, designate products obtained by forming a primary product, whose length is notably greater than their width and thickness, in the case of which, however, the width and thickness are usually in comparable orders of magnitude. Typical examples of long products are rods, bars, profiles and the like.

In the case of dynamically highly stressed components, such as e.g. connecting rods for combustion engines, its moved mass is particularly significant, in addition to the strength or stiffness of the respective component.

In order to implement cost-effective lightweight construction solutions for such applications, lightweight construction steels based on iron with high Al contents have been proposed. These are far above the Al contents which are present in steels in the case of which Al is added only for oxidation in the course of the steel production. G. Frommeyer, E. J. Drewes and B. Engl give an overview concerning such steels in “Physical and mechanical properties of iron-aluminium-(Mn, Si) lightweight steels”, Revue de Métallurgie, 97, pp. 1245-1253, October 2000, doi: 10.1051/metal:2000110.

In addition to molybdenum and chromium, aluminium belongs to the elements that have a ferrite-stabilising effect and can even suppress the austenite-ferrite conversion entirely.

The known density-reduced steel materials can be classified roughly into four groups:

Group 1: Steels with aluminium contents of up to 25 wt %, carbon contents of up to 2.5 wt % and manganese contents of up to 40 wt %. Steels composed in this manner have a convertible structure and are used to produce components such as connecting rods or roll bearings generated by hot forming (R. L. Bülte, Dissertation: Untersuchung von hochaluminiumhaltigen Kohlenstoffstählen auf ihre Eignung als Wälzlagerwerkstoff, Aachen, 2008). The principle underlying this material group has already been long since known. In this case, Al contents of 4.0-25.0 wt %, combined with contents of C of 0.20-2.0 wt %, Mn of 8.0-40.0 wt %, Si of up to 3.0 wt %, N of up to 1.0 wt % and Nb of up to 4.0 wt % are considered ideal group ranges (U.S. Pat. No. 1,892,316 A, DE 12 62 613 B, DE 10 2006 030 699 A1, DE 10 2005 027 258 A1, DE 10 2010 012 718 A1).

Group 2: Steels with aluminium contents of up to 12 wt % and manganese contents of up to 50 wt % to also ensure an austenite/(ferrite, bainite, martensite) conversion. Steels of this type are used as sheet in body work, container and pipe construction (DE 102 31 125 A1, DE 103 59 786 A1, DE 196 34 524 A1, EP 2 767 601 A1).

Group 3: Ferritic steels with aluminium contents of up to 23 wt % and chromium contents of up to 35 wt % to achieve anticorrosion properties via a cover layer formation. These steels are used in the field of automotive engineering, facade cladding, in the case of chemical apparatuses, in the case of combustion engines and in exhaust gas systems (DE 10 2009 031 576 A1, DE 100 35 489 A1, DE 10 2010 006 800 A1, DE 10 2007 047 159 A1, DE 10 2007 056 144 A1, DE 12 08 080 B, GB 2 186 886 B).

Group 4: Stainless austenitic and multiphase steels with up to 10 wt % aluminium, up to 30 wt % manganese and up to 18 wt % chromium. Manganese serves here as an austenite stabiliser against the elements Al and Cr acting in a ferrite-stabilising manner (DE 10 2005 024 029 B3, DE 10 2005 030 413 B3, DE 199 00 199 A1).

The alloy concepts associated with group 1 lead necessarily to the formation of an iron aluminium carbon phase which is also designated in technical language as a kappa-carbide. Kappa-carbides increase strength only to a limited extent, but impair the toughness properties owing to the preferred precipitation at the grain boundaries.

In light of the background of the previously mentioned prior art, the object of the invention was to indicate a density-reduced material based on iron, whose mechanical properties make it suitable for a wide range of applications in particular in the area of the automobile industry.

A method is also indicated by means of which flat or long products made of steels of the type in question here can be produced in an operationally-safe and economic manner.

In relation to the material, the invention achieved this object with the steel indicated in Claim 1.

In relation to the method, the invention achieved the above-mentioned object in that the work steps indicated in Claim 9 are used when processing steels according to the invention into flat or long products.

Advantageous configurations of the invention are indicated in the dependent claims and are explained below in detail as is the general inventive concept.

In the case of the alloy according to the invention, in addition to the known solid solution strengthening elements chromium, molybdenum, silicon and manganese, the required strength of more than 500 MPa is formed via precipitation phases. These phases are largely precipitated in an intracrystalline manner. Strength-increasing intermetallic phases, such as the laves phase, consist substantially of iron, titanium and optionally molybdenum, Ni(Mn, Al, Ti), Ni₂MnAl, Ni₃Ti and Cu. Fine carbides, fine nitrides and fine carbonitrides also, however, provide a contribution to the strength level.

In order to realize the reduction of density by alloying with aluminium without kappa-carbide precipitation, in the case of the alloy concept according to the invention alloying with carbon is largely dispensed with and freedom of conversion accepted.

To avoid coarse carbides, nitrides or carbonitrides, in the case of the steel according to the invention, the carbon and nitrogen contents are instead limited to values that are as low as possible such that at most isolated carbides or carbonitrides occur during solidification.

For this purpose, the C content of the steel according to the invention is at most 0.2 wt %. The occurrence of undesired carbides can be particularly safely prevented when the C content is less than 0.1 wt %, in particular at most 0.02 wt % or at most 0.01 wt %.

Similarly, to avoid the occurrence of nitrides, the N content is limited to at most 0.020 wt %, in particular at most 0.005 wt %.

The Al content of steels according to the invention is 6 to 25 wt %, in particular at least 10 wt %.

Without corresponding counter measures, deteriorations to the mechanically technological properties and poor forming behaviour would occur from a Al content of more than 12 wt % and indeed caused by a superlattice D0₃ (Fe₃Al) forming in the structure or product precursor of a near-order (system FeAl). The sufficient addition of contents of manganese, silicone, chromium, molybdenum, vanadium, tungsten, nickel, niobium or titanium can counteract these effects. For this purpose, the invention provides, in the case of Al contents of more than 12 wt %, that the contents of Cr, Mo, Mn, Si, V, W, Ni, Nb, Ti meet the following condition:

(% Cr+2*% Mo+% Mn+% Si+% V+% W+% Ni+% Nb+% Ti)>0.05*% Al

with % Cr: Cr content of the steel, % Mo: Mo content of the steel, % Mn: Mn content of the steel, % Si: Si content of the steel, % V: V content of the steel, % W: W content of the steel, % Ni: Ni content of the steel, % Nb: Nb content of the steel, % Ti: Ti content of the steel and % Al: Al content of the steel.

It has been proven to be advantageous in this case for 0.1 3.5 wt % Si, in particular up to 1.5 wt % Si to be present in the steel according to the invention. In this case, a particularly safe effect of the presence of Si results when the Si content is at least 0.20 wt %.

Sulphur can be added to the steel according to the invention to improve its machinability in contents of up to 0.40 wt %, optimal effects resulting in the case of contents of up to 0.28 wt %. To safely utilise the positive influence of the presence of S, the S content of a steel according to the invention can be set to at least 0.01 wt %.

The strength of the material can be set by the targeted addition of up to 10 wt % Ti. In this case, this effect of Ti can be particularly safely achieved as a result of at least 0.60 wt % Ti being present in the steel according to the invention. Optimal effects of Ti result when the Ti content is at least 0.90 wt % or at most 2.0 wt %.

Chromium in contents of up to 6.0 wt % contributes to the prevention of the superlattice D03 and to solid solution strengthening. To safely utilise the favourable influences of Cr in the steel according to the invention, the Cr content can be set to at least 0.30 wt %. Optimal effects result in this case when at least 0.50 wt % or at most 3.5 wt % Cr are present in the steel according to the invention.

Mo in contents of up to 3.0 wt % helps the prevention of the superlattice D03, contributes to solid solution strengthening and supports the formation of desired precipitations. To safely achieve this, the Mo content can be set to at least 0.1 wt %, wherein optimal effects of the presence of Mo occur in the steel according to the invention when its Mo content is at least 0.25 wt % or at most 2.8 wt %.

If V is present in contents of up to 1.0 wt % in the steel according to the invention, the superlattice D03 can also be prevented. To safely achieve this, the V content can be set to at least 0.10 wt %, wherein optimal effects of the presence of V occur in the steel according to the invention when its V content is at least 0.20 or at most 0.50 wt %.

Tungsten in contents of up to 1.0 wt % also acts positively on the prevention of the superlattice D03. To safely utilise the favourable influences of W in the steel according to the invention, the W content can be fixed to at least 0.20 wt %. Optimal effects result in this case when at least 0.40 wt % or at most 1.0 wt % W is present in the steel according to the invention. If W is supposed to be added as an alternative to Mo, double as much tungsten as molybdenum must be added to achieve the same effect.

In the steel according to the invention, copper in contents of up to 4 wt % causes the strength to increase via copper precipitations. This effect can be safely achieved as a result of the Cu content being at least 0.5 wt %, wherein contents of at most 3.50 wt % have been found to be particularly positive. To ensure the hot formability, roughly the same amounts of nickel should be alloyed to the material.

The addition of up to 0.08 wt % boron can suppress the precipitation behaviour of the hardness-increasing phases at the grain boundaries in the steel according to the invention. This can be safely achieved by at least 0.0005 wt % B being present in the steel according to the invention. B contents of more than 0.08 wt %, in contrast, impact negatively on the formability of the steel. To safely prevent this, the B content of the steel according to the invention can be limited to at most 0.0030 wt %.

If Nb is present in contents of up to 1.5 wt % in the steel according to the invention, Nb similarly contributes to the prevention of the superlattice D03 and strength-increasing precipitation phases are formed. To safely achieve this, the Nb content can be set to at least 0.05 wt %, wherein optimal effects of the presence of Nb occur in the steel according to the invention when its Nb content is at least 0.10 wt % or at most 0.30 wt %.

The structure matrix of the steel according to the invention consists largely, i.e. at least 85 vol %, of ferrite, wherein higher ferrite contents of at least 90 vol % may be particularly favourable.

An austenite proportion of up to 10 vol % in the structure can, however, also positively impact the toughness of the steel. Therefore, it may be expediently for the alloy of the steel according to the invention to be set such that at least 2 vol % austenite is present in the structure of the steel. If the austenite proportion is greater than 10 vol %, this negatively impacts the precipitation behaviour of the intermetallic phases.

In the case of the remaining structure components not occupied by ferrite or austenite, they are contents of intermetallic phases and proportions of carbide, nitride, bainite or perlite. The proportions of these remaining components in the structure of the steel according to the invention are, however, so low that they have at best insignificant impacts on its properties.

Undesired austenite proportions exceeding 10 vol % can be prevented by suitable setting of the Mn and Ni contents of the steel according to the invention.

For this purpose, the Mn content of a steel according to the invention is limited to at most 3.5 wt % and the Ni content to at most 4.0 wt %. The positive influence of Mn and Ni on the quality of the steel according to the invention can be optimally utilised when the sum of the contents of Mn and Ni is at most 5 wt %. It has been proven to be particularly advantageous when the Mn content is set to at most 1.0 wt % or the Ni content to at most 1.5 times the optionally present copper content. The positive influences of the presence of Mn or Ni, such as the maintenance of optimal mechanical properties enabled by the targeted addition of Ni or Mn, can be particularly utilised in the steel according to the invention as a result of the Mn content of the steel being at least 0.20 wt %.

Negative impacts of the S content specifically approved according to the invention can be prevented as a result of the ratio % Mn/% S of the manganese content % Mn to the sulphur content % S being set to more than 2.0.

The method according to the invention for producing a flat steel or long steel product comprises at least the following work steps:

a) providing a primary product consisting of a steel formed according to any one of the preceding claims, such as a slab, a flat slab, a billet or a cast strip,

b) heating the primary product to a hot forming temperature of 700-1280° C.,

c) hot forming the primary product heated to the hot forming temperature into the flat steel or long steel product.

A complete solution of any present precipitations, reasonable forming forces, a sufficient recrystallisation kinetic and a minimal grain growth are achieved by hot forming in the temperature range of 700-1280° C. The hot forming temperature is optimally 850 to 1050° C. In the case of forming in the temperature range between 850° C. and 1050° C., a particularly fine grain structure, grain size according to ASTM E 112=4 and finer is achieved.

After hot forming, the flat or long product obtained according to the invention can undergo different heat treatments to set its mechanical properties.

One method of such a heat treatment that is advantageous in terms of the energy utilisation can consist of the flat steel or long steel product obtained after hot forming being cooled slowly following the hot shaping at a cooling speed of max. 3.0 K/min, in particular 1.5 K/min, wherein the cooling speed should not be less than 1.0 K/min from a process-economical point of view. In this manner, the end strength of the steel is achieved directly by precipitation of the precipitation phases such as e.g. Laves, Heussler, copper, Ni3Ti and/or Ni3Al phases. This approach is particularly advantageous when the Ti content of the steel according to the invention is more than 0.60 wt %. The toughness of the flat or long product thus obtained is typically in the range of 700-1150 MPa.

It may be advantageous for the flat or long product hot formed from the steel according to the invention to firstly undergo a solution annealing at more than 700° C., in particular 700-1250° C. or 700-1000° C. and to subsequently at a cooling speed of at least 25 K/min to suppress the formation of precipitations. After the respective cooling, an intermediate product is present which is comparably soft and easily mechanically processable with a tensile strength of less than 900 MPa.

After the respective cooling, the product obtained can be aged at temperatures of 150-700° C. for a period of 15 minutes to 30 hours in order to positively influence the precipitation state of its structure. In the case of Ti-containing variants of the steel according to the invention, a precipitation of the Ti-containing precipitation phases results which in particular cause a strength increase.

The invention will be explained further below by exemplary embodiments.

Example 1

A steel S1 with the composition indicated in Table 1 was melted and cast to form a block. This primary product has been heated to a hot forming temperature of 1050° C. and formed by pressing at this temperature to a semi-finished product (long product).

The product obtained in this manner has been solution-annealed at a solution annealing temperature of 1050° C. for a period of 1 h and subsequently quenched by immersing in water.

After quenching, the steel had a tensile strength of 800 MPa and with this comparably low strength could be processed in a simple manner by machining.

After the mechanical processing, the processed product was aged to set its end strength at 500° C. for 4 hours. After this ageing, the steel of the product had a strength of 1070 MPa. It was shown that the ageing treatment led to at best minimal warping of the product. Ageing at a temperature of 550° C. and for a period of 1 hour resulted in a strength of 1200 MPa. A strength of 1300 MPa could be achieved at a temperature of 600° C. and the same ageing period of 1 hour.

The density of the steel S1 used in example 1 was 6.9 kg/dm³.

Its structure consisted of more than 99 vol % of ferrite and precipitated phases. The precipitated phases are extremely fine and usually not discernible in an optical microscope.

Example 2

A steel S2 with the composition indicated in Table 1 was melted and cast to form a block. The primary product in question has been formed by pressing at a hot forming temperature of 1050° C.

The product obtained in this manner has been solution-annealed at a solution annealing temperature of 1050° C. for a period of 1 h and subsequently quenched by immersing in water.

After quenching, the steel had a tensile strength of 920 MPa and with this comparably low strength could be mechanically processed in a simple manner.

After the mechanical processing, the product was aged at 500° C. for 4 hours to set its end strength. After this ageing, the steel of the product had a strength of 1175 MPa. It was shown here as well, that the ageing treatment led to at best minimal warping of the product.

The density of the steel S2 used in example 2 was 6.9 kg/dm³. Its structure consisted of more than 99 vol % of ferrite and precipitated phases.

Example 3

A steel S3 with the composition indicated in Table 1 was melted and cast to form a block.

The primary product in question has been formed into a block by pressing at a hot forming temperature of 1000° C.

The product obtained in this manner has been solution-annealed at a solution annealing temperature of 1075° C. for a period of 1 h and subsequently quenched by immersing in water.

After quenching, the steel had a tensile strength of 860 MPa and with this comparably low strength could be mechanically processed in a simple manner.

After the mechanical processing, the product was aged at 500° C. for 1 hour to set its end strength. After this ageing, the steel of the product had a strength of 1540 MPa. It was shown that the ageing treatment led to at best minimal warping of the product.

The density of the steel S3 used in example 3 was 6.7 kg/dm³.

Its structure consisted of more than 99 vol % of ferrite and precipitated phases.

Example 4

A steel S4 with the composition indicated in Table 1 was melted and cast to form a block. Chromium and molybdenum were added to the melt to avoid an adverse superlattice (D03) and for solid solution strengthening.

The primary product in question has been formed by pressing at a hot forming temperature of 1075° C.

The product obtained in this manner has been solution-annealed at a solution annealing temperature of 1050° C. for a period of 1 h and subsequently quenched by immersing in water.

After quenching, the steel had a tensile strength of 805 MPa and with this comparably low strength could be mechanically processed in a simple manner.

The product was aged at 550° C. for 1 hour to set its end strength. After this ageing, the steel of the product had a strength of 1260 MPa. It was shown that the ageing treatment led to at best minimal warping of the product.

The density of the steel S4 used in example 4 was 6.1 kg/dm³.

Its structure consisted of more than 99 vol % of ferrite and precipitated phases.

TABLE 1 Steel Al Ti C Si Mn N Cr Mo S1 8 1.25 0.04 0.59 0.56 0.001 S2 8 1.25 0.01 1.43 1.50 0.002 S3 10 2.15 0.01 0.51 0.49 0.002 S4 18 1.31 0.01 0.5 0.47 0.002 2 0.24 Information in wt %, remainder iron and unavoidable impurities 

1. A steel with a density of less than 7.25 kg/dm³ and comprising (in wt %) C: up to 0.20% Si: 0.1-3.50% Mn: 0.1-3.50% N: up to 0.020% S: up to 0.40% P: up to 0.009% Al: 6.0-25.0% Ti: 0.55-10.0% Cr: up to 6.0% Mo: up to 3.0% Ni: up to 4.0% V: up to 1.0% W: up to 1.0% Cu: up to 4% B: up to 0.08% Nb: up to 1.5% remainder iron and unavoidable impurities caused during production, wherein the structure of the steel has more than 85 vol % ferrite and up to 10 vol % austenite and as the remainder contents of intermetallic phases and proportions of carbide, nitride, bainite or perlite.
 2. The steel according to claim 1, wherein the C content is less than 0.02 wt %.
 3. The steel according to claim 1, wherein % Mn/% S>2.0 applies for the ratio % Mn/% S of the Mn content % Mn and the S content % S.
 4. The steel according to claim 1, wherein the sum of the contents of Ni and Mn is at most 5 wt %.
 5. The steel according to claim 1, wherein the N content is at most 0.005 wt %.
 6. The steel according to claim 1, wherein the Al content is at least 10 wt %.
 7. The steel according to claim 6, wherein the Al content is more than 12 wt % and the contents of Cr, Mo, Mn, Si, V, W, Ni, Nb, and Ti meet the following condition: (% Cr+2*% Mo+% Mn+% Si+% V+% W+% Ni+% Nb+% Ti)>0.05*% Al with % Cr: Cr content of the steel, % Mo: Mo content of the steel, % Mn: Mn content of the steel, % Si: Si content of the steel, % V: V content of the steel, % W: W content of the steel, % Ni: Ni content of the steel, % Nb: Nb content of the steel, % Ti: Ti content of the steel, % Al: Al content of the steel.
 8. The steel according to claim 1, wherein the B content is at least 0.0005 wt %.
 9. A method for producing a flat steel or long steel product comprising: a) providing a primary product consisting of a steel formed according to claim 1, wherein the primary product is a slab, a flat slab, a billet or a cast strip; b) heating the primary product to a hot forming temperature of 700-1280° C.; and c) hot forming the primary product heated to the hot forming temperature into the flat steel or long steel product.
 10. The method according to claim 9, wherein the hot forming temperature is at most 1000° C.
 11. The method according to claim 9, wherein the flat steel and long steel product obtained is cooled following the hot forming at a cooling speed of at most 3 K/min.
 12. The method according to claim 9, wherein the Ti content of the steel is at least 0.60 wt % and in that the flat steel or long steel product obtained after the hot forming is cooled either in a first heat treatment step directly from the hot forming or after solution annealing at a temperature of more than 700° C. at a cooling speed of at least 25.0K/min.
 13. The method according to claim 12, wherein the flat steel or long steel product is aged in a further heat treatment step at temperatures of 150-700° C. for a period of 15 mins to 30 hours.
 14. The method according to claim 13, wherein the flat steel or long steel product is mechanically processed between the first heat treatment step and the further heat treatment step. 